1. Field of the Invention
The present invention relates to a stencil printer for printing an image on a sheet via a master wrapped around a print drum.
2. Description of the Background Art
A thermosensitive stencil for use with a stencil printer has a laminate structure made up of a 1 xcexcm to 8 xcexcm thick, thermoplastic resin film and a porous base adhered to one side of the resin film. The porous base is formed of Japanese paper, synthetic fibers or a mixture thereof.
A digital stencil printer includes a thermal head or similar heating means that perforates, or cuts, the film surface of the stencil with heat in accordance with digital image data representative of a document image. After the perforated stencil, i.e., a master has been wrapped around a print drum, ink is fed from the inside of the print drum while a press roller or similar pressing member presses a sheet against the print drum. As a result, the ink is transferred from the print drum to the sheet via the perforations of the master.
Assume that the heating means is implemented as a thermal head. Then, a platen roller, which faces the thermal head, is rotated to convey the stencil positioned between the heating surface of the head and the platen roller. Generally, a pressing mechanism presses the thermal head against the platen roller to thereby generate platen pressure, which presses the stencil against the heating surface of the thermal head.
Thermosensitive stencils in general are classified into some different kinds by the thickness of the thermoplastic resin film, the material of the porous base, the kind and the amount of an anti-sticking agent or an antistatic agent coated on the side of the film to be perforated and so forth. Each stencil printer, strictly a master making device included therein, has heretofore been operable only with a particular kind of stencil.
More specifically, when different kinds of stencils are applied to a single master making device, a conveying distance differs from one stencil to another stencil and effects the reproducibility of the size of an image, as well known in the art. This is because slip between the film surface of the stencil and the surface of the thermal head and friction to act between the porous base of the stencil and the platen roller depend on the kind of the stencil. Further, a load to act during perforation due to a master making speed and image density also has influence on the reproducibility of an image size. In addition, the front tension and back tension of the stencil effect the reproducibility of an image size. When such factors are brought out of balance, the stencil conveying distance varies due to changes in slip, friction and load.
The degree of slip varies in accordance with the surface configuration of the thermal head, e.g., the material and smoothness of a protection film and the material of the porous base adhered to the stencil. Other factors that effect slip include the kind and the amount of the anti-sticking agent, antistatic agent or similar overcoat agent coated on the film of the stencil, the material and the amount of a filler contained in the film, and the thickness of the film. The anti-sticking agent promotes slip between the surface of the thermal head and the film while the antistatic agent reduces charging to occur during the conveyance of the stencil.
The degree of friction varies in accordance with the material, surface configuration, rubber hardness and other factors of the platen roller and the kind of the porous support. Other factors that effect friction include the kind and density of the porous base, the kind and the amount of an overcoat agent contained in the base, and the amount of an overcoat agent, which is coated on the film surface, migrated from the film surface to the base when the stencil is rolled up.
A load increases with an increase in image density on a single line and with an increase in master making speed. Further, a load is proportional to the front tension and back tension of the stencil.
When a single master making device conveys a stencil, the thickness of the stencil and the amount of crush of the stencil ascribable to pressure have influence on the conveying distance, too.
Another factor that effects the conveying distance is the environmental conditions. For example, when ambient temperature rises, the diameter of the platen roller increases due to thermal expansion and causes the peripheral speed of the roller to vary. Particularly, when the porous base is hygroscopic, friction to act between the platen roller and the base varies in accordance with humidity and also effects the conveying distance.
The prerequisite with master making is that the thermal head surely perforates the film of the stencil by melting it with heat. Close adhesion between the film surface and the heating elements of the thermal head is one of various factors having influence on the perforation condition. The degree of close adhesion determines a perforation condition and sometimes leaves the film left unperforated. As for the printer body, irregularity in the amounts of heat generated by the heating elements of the thermal head, platen pressure and the surface configuration of the platen roller effect close adhesion.
Specifically, assume that a single master making device with a fixed platen pressure operates with a stencil that cannot be desirably perforated without resorting to high platen pressure and a stencil that can be done so even at low platen pressure. Then, the platen pressure must be matched to the former kind of stencil, but such a platen pressure is excessively high for the latter kind of stencil. The excessive platen pressure causes more than a necessary mechanical stress to act on the thermal head and is not desirable from the standpoint of durability, e.g., wear resistance of the thermal head.
Further, a greater amount of adhesive for adhering the film and porous base must be used when the platen pressure is high than when it is optimum (low); otherwise, the film and base would separate from each other when conveyed between the thermal head and the platen roller. This not only wastes the adhesive, but also adversely effects the perforation condition.
Assume that the same energy is applied to the thermal head when different kinds of stencils are used. Then, the perforation condition sometimes differs and sometimes remains the same, but is not optimum, depending on so-called stencil (film) sensitivity that is determined by the material, thickness and so forth of the film.
To reduce offset particular to a stencil printer, the perforation diameter of the film should preferably be small although the density of a print should be taken into account. However, when porous base has low ink permeability, the perforation diameter of the film must be large enough to transfer a sufficient amount of ink to a sheet; otherwise, the resulting image density would be short.
Master making conditions differ from one kind of stencil to another kind of stencil, as stated above. Therefore, when the user selects a particular kind of stencil by attaching importance to, e.g., image quality or the cost of the stencil itself, the user must vary the various conditions of the master making device one by one in matching relation to the kind of the master. This cannot be done without resorting to expertness or troublesome work. This is why the user has heretofore been obliged to use only a stencil matching with conditions set at the time of delivery.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 11-115145, 11-115148, 6-320851, 8-090747, 9-277686, 11-020983, and 11-091227.
It is an object of the present invention to provide a stencil printer capable of easily, automatically setting master making conditions matching with a desired kind of stencil, and promoting diversification from the user standpoint.
A stencil printer of the present invention perforates, or cuts, a thermosensitive stencil with a thermal head to thereby make a master. The stencil printer includes a stencil distinguishing device for automatically identifying the kind of the stencil or a master setting device for allowing the operator of the printer to set the kind of the stencil. An adjusting device selects, among master making conditions experimentally determined beforehand, a master making condition matching with information output from the stencil distinguishing device or the stencil setting device. The operator can easily change the master making condition in accordance with the kind of a stencil to use.